First, watch this video.
Before you do, look at your hand and take notice of how you are holding your mouse.
http://www.youtube.com/watch?v=luCxbVslSFM
From there, each mouse manufacturer will somewhat split their lineups into the different grips, find out which you are and look into them. Razer, Steelseries, Logitech, Coolermaster and Thermaltake are some big names in peripherals and are where I would start looking. Find one that suits your grip and has features you want (bajillion thumb buttons for MMO gaming, lighting because its cool, high DPI for twitch shooters, etc).
I personally use a Razer Deathadder 2013 and love the thing.